I've got 3 sigma lens.18-200mm , 105mm macro and 10-20mm . All are excelent.Have had no problem with neither of them . I test focusing on all my lens using a technique I learn in the internet downloading a focusing sheet.And check for out of center just magnifying to 100% and carefully pixel peeping to see if I see differences between one side and the other...Up to now, no problem.

My experience with Sigma lenses are just mine.Be adviced , I've read about others having different experiences then mine having had to get up to 3 copies of the same lens to get a decent one.

It could be that when a lens comes out on the market they some times have some bugs in them that afterward are fixed in the ongoing production.Be careful buying a new on the market Sigma lens.If posble check it out before buying at the store and be sure of the stores policy regarding taking it back if its defective .